sci.med.vision: Frequently Asked Questions (FAQ), Part 2/5
Grant Sayer
Archive-name: vision-faq/part2
Posting-Frequency: monthly
Version: $Id: faq-part2.txt,v 1.25 1995/11/22 22:38:38 grants Exp $
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FREQUENTLY ASKED QUESTIONS: Vision and EyeCare
Part 2/5
(Copyright (C), 1995 Grant Sayer)
+============================================================================+
+============================================================================+
+ Section 1: Optics of the Eye and General Information +
+============================================================================+
1.1: What is Myopia ?
---------------------
Myopia is often referred to as "short-sightedness" or "near-sighted".
An eye is myopic when the "far point"; a point at which light from an object
is focussed on the retina, is located at a finite distance in front of the eye.
Myopia can be due to either an eye which is too long relative to the optical
power of the eye (axial myopia), or because the optical power of the eye is
too high relative to the length of the standard eye (refractive myopia). The
focus is correctly adjusted with a "minus" power lens, or concave lens.
WWW resources:
URL: http://www.eyenet.org
- American Academy of Ophthalmology.
URL: http://www.ozarksol.com/medsource
- contains nice pictures explaining the focal point for each
condition
URL: http://www.west.net/~eyecare/myopia.html
- details and pictures to explain myopia.
URL: http://www.execpc.com/~warren/optweb/errors.html
- John Warren OD home page
URL: http://www.web-xpress.com/vhsc/eyedz.html
- informational brochures on eye conditions
1.2 What is Hyperopia ?
-----------------------
Hyperopia is often referred to as "long-sightedness" or "far-sighted".
An eye is hyperopic when the far point is at a virtual point behind the eye.
Generally the hyperopic eye is too short with respect to the refractive state
of the standard eye (ie an emmetropic eye or eye requiring no optical
correction) or because the optical power of the eye is too low relative to
the length of the standard eye. The focus is correctly adjusted using a
"plus" lens power or convex lens.
WWW resources:
URL: http://www.eyenet.org
- American Academy of Ophthalmology.
URL: http://www.ozarksol.com/medsource
- contains nice pictures explaining hyperopia and showing the
focal point of image formation in this condition
URL: http://www.west.net/~eyecare/hyperopia.html
- details and pictures to explain the condition.
URL: http://www.execpc.com/~warren/optweb/errors.html
1.3 What is Emmetropia ?
------------------------
Emmetropia is just another name for an eye that has no optical defects and
a precise image is formed on the retina.
1.4 What is Astigmatism ?
-------------------------
An astigmatic eye generally has two different meridians, at 90degrees to
each other, which cause images to focus in different planes for each meridian.
The meridians can each be either myopic, hyperopic or emmetropic.
The correction for astigmatism is a lens power at a particular direction
of orientation [ see section 4.1 ]
Astigmatism causes images to be out of focus no matter what the distance.
It is possible for an astigmatic eye to minimise the blur by accommodating,
or focusing to bring the "circle of least confusion" onto the retina.
URL: http://www.west.net/~eyecare/astigmatism.html
- details and pictures to explain the condition.
URL: http://www.web-xpress.com/vhsc/astigm.html
- diagrams and questions and answers about the causes, symptoms
of astigmatism.
URL: http://www.sna.com/etonline/vision
- still under construction but contains a large amount of eyecare
information
1.5 What is Presbyopia ?
------------------------
Presbyopia describes the condition whereby the amplitude of accommodation,
or ability to focus on objects at near, decreases with increasing age.
It is corrected by a different prescription for reading, which is additive to
the normal spectacle correction used for distance vision.
Some recent research indicates that presbyopia may be caused by structural
changes in the tendons and elastic fibres of the posterior ciliary body. The
age related increase in fibrillar material could cause decreased compliance
of the posterior insertion of the ciliary muscle.
For more details see
Tamm E., Lutjen-Drecoll E., Jungkunz W., & Rohen J.W., "Posterior attachment
of ciliary muscle in young, accommodating old presbyopic monkeys."
Invest. Ophthal & Vis. Sci. 1991 Apr;32(5):1678-92
URL: http://www.west.net/~eyecare/presbyopia.html
1.6 How is Visual Acuity Measured
----------------------------------
Visual acuity is the measure of the sensitivity of the visual system.
It is expressed in Snellen notation, expressed as a fraction, where the
numerator indicates the test distance and the denominator denotes the
distance at which the letter read by the patient subtends 5 minutes of arc.
Normal vision is expressed as 20/20 (or 6/6 in countries where metric
measurements are used).
An acuity of 20/60 means that the patient was tested at 20feet but could
only see letters that a person with normal vision could read at 60feet.
Other WWW references:
URL: http://www.west.net/~eyecare/visual_acuity.html
+============================================================================+
+ Section 2: Spectacles +
+============================================================================+
2.0 Terminology of Spectacles
-----------------------------
Lenses commonly used for spectacles are either spherical powered or
cylindrical powered, or a combination of the two. A spherical lens is
one that focus a distant object to a point of focus. As in the
'crude' ASCII image below the distant parallel light is focused at F1
/\
.......>..........||..
|| .
|| .
|| . F1
|| .
|| .
.......>..........||..
\/
A cylindrical lens has 2 focal points corresponding to the two different
powers of the lens. Distant light is focused at two focal planes, labelled
F1 and F2 in the diagram below. The diagram shows the axes of the
cylindrical powers at 90degrees to each other.
^
------------|--------------
| l -
............|...l -
| l . -
+ .F1 - F2
l | . -
..............l . |. -
l | -
------------|-------------
V
Other WWW references:
URL: http://www.west.net/~eyecare/optical_lens_prescriptions.html
URL: http://www-sci.lib.uci.edu/~martindale/Medical.html#Oph
Contains references to other web sites on glasses and how they work
URL : http://www.nbn.com/youcan/glasses/eyeglasses.html
(Questions and answers on spectacles and glasses)
2.1 How to read a glasses prescription
--------------------------------------
A prescription for spectacles will generally contain the powers for lenses
of each eye, possibly also a reading addition power, information on the
separation of the lenses and any special requirements for the lenses (eg,
type of lens, tinting, coatings etc). Lens power is written in Dioptres,
which is 1/focallength(metres). For example, a 5.0D (5.0 dioptre) lens
has a focal length of 0.2m or 200mm
An example of a spectacle correction is -
Patient Name <Person F Bar>
OD: +1.50/-1.00 x 35
OS: +1.75/-1.25 x 135
Add: +1.50
PD 62
The terms mean
OD - oculus dextrum or right eye
OS - oculus sinistrum or left eye
Add - Near addition
PD - interpupillary distance
The prescription values are read, firstly for the right eye
+1.50 - spherical power (positive)
-1.00 x 35 - cylindrical lens of negative power with axis at 35 degrees
Note: There may be variations in the way that the prescription is written
since there are different standards for the way cylinders are
represented and the coordinate system of the axes.
The coordinate system for the prescription is 0 to 180 degrees with 90 at
the vertical or 12 o'clock position.
RIGHT EYE LEFT EYE
90 90
| |
135 | 45 135 | 45
\ | / \ | /
\ | / \ | /
\ | / \ | /
\|/ \|/
180 ======+====== 0 < NOSE > 180 ======+====== 0
Other WWW references:
URL: http://www.west.net/~eyecare/optical_lens_prescriptions.html
2.2 Why the difference between the way Optometrists and Ophthalmologists
write the prescription ?
-----------------------------------------------------------------
There are two ways of writing the prescription; referred to as "plus-cyl"
form or "minus-cyl" form. The plus cyl form, sometimes used by
Ophthalmologists is written
+1.50 D
---------------
+0.50 x 35
The equivalent minus cyl form, generally used by optometrists is written
+2.00 D
---------------
-0.50 x 125
2.3 How to convert between the two forms
-----------------------------------------
To convert from "plus-cyl" form to "minus-cyl" form use the following steps:
1. Add the cylinder power to the sphere
2. Change the sign of the cylinder from + to -
3. Add 90 degrees to the axis if the original axis is less than 90, or
4. Subtract 90 degrees from the axis if the original is greater than 90.
To convert from "minus-cyl" to "plus-cyl" form the steps are reversed, i.e
1. Add the cylinder power to the sphere power
2. Change the sign of the cylinder from - to +
3. Add 90 degrees to the axis of the cylinder if axis < 90
4. Subtract 90 degress from the axis if axis > 90
Other WWW references:
URL: http://www.west.net/~eyecare/optical_lens_prescriptions.html
2.4 Eyeglass Lens materials
--------------------------------------------
Spectacle lenses are made of two main types of materials - plastic or glass.
Plastic lenses are often CR39 or polycarbonate. Glass lenses come in a
variety of refractive indexes, designed to minimise the thickness.
HiIndex lens materials are commonly 1.56, 1.60 and 1.67, as compared to
XXXX for common crown glass.
Ultimately, the lens choice is very much a user application. Hi Index
will provide a thinner edge and better cosmetic appearance but still
weigh in heavy. Comparatively, plastic lenses are lighter but in
high minus prescriptions will have a significantly thicker lens edge.
Furthermore the size of the frame will influence the overall edge thickness
since a larger eyesize means a larger lens.
2.5 Types of Spectacle Lenses
------------------------------
+ Single Vision lenses
- spherical or with astigmatic correction
- in plastic or glass
+ Bifocal Lenses
- made of a main lens and an additional segment for near vision
- variety of shapes of near addition
+ Trifocal Lenses
- composed of a main lens and two reading ssegments; one for near
and one for intermediate distances (about arm lengths)
+ Multifocals
- composed of main lens and continuously variable addition for
various near reading positions
+ Aphakic Lenses
- high power plus lenses used for cataract patients.
- made with aspheric surfaces to minimise aberrations due to shape of
the lens
+ Photochromic lenses
- darken when exposed to short wavelength radiation (300nm - 400nm)
or ultra-violet light
- rate of darkening and final transmission of the lens (saturation
transmission) depends on the ambient temperature.
- lenses will not become as dark when driving due to UV absorption
by the car windscreen.
- available in plastic lenses now - Transitions+ and Spectralite are
two common lens names.
+ Coated Lenses:
- spectacle lenses may often be treated with a surface coating
(vacuum coating) to reduce unwanted reflections.
- the surface reflection is related to the material index so that
different lens materials will have different amounts of surface
reflectance.
- reflections are classified as 4 main types
(a) reflections visible by an observer from the front
(b) internal reflections of the lens itself
(c) reflections from behind the lens, eg overhead lighting
(d) reflections from the corneal surface.
+ Hi Index Lenses:
- manufactured from a higher refractive index material that enables
the lens thickness to be reduced whilst still maintaining the optical
properties. That is the optical power of a lens is a combination of
the two surfaces, front and back, and the thickness of the lens.
- increased index of lens also means that there is a reduction in the
base curve (back curve) so that there is greater thickness savings
when comparing two lenses of the same centre thickness.
+ Toughened Lenses
- Either heat or chemical treated to increase impact strength
- Used in industrial applications for added safety and protection.
+ Aspheric Lenses
- elliptical curve surfaces designed to minimise lens abberations
available as single vision and even some in progressive lens
designs.
+ Polarized Lenses
- available in glass, plastic and hi-index plastic (n=1.56)
2.6 Ultra Violet absorption and Lenses
--------------------------------------
Spectacle lenses, depending on the type of material will absorb varying
amounts of ultraviolet light. In order of best absorbing lens the materials
are polycarbonate, plastic and finally glass lenses. Different standards
define the amount of absorption required, and/or the definition of the
categories for spectacle lenses. The amount of UV absorption is also
influenced by tinting, dying and lens coatings.
2.7 Standards Requirements for Spectacle Lenses
-------------------------------------------------
International and local standards define the properties of lenses that
are required for spectacle lenses. There are different standards for
prescription spectacle lenses and sunglass or fashion lenses.
In the USA the standards include -
-U.S. Sunglass standard for durability is Ct. SGSTD40 (I think)
-U.S. Standard for tints and UV requirements ANSI Z80.3
You can search the ANSI Standards from the WEB using the ANSI homepage
URL: http://www.ansi.org
The standard information is located at the ANSI site as well as other
useful information on opththalmic and optometric standards
.
+ Prescription Lenses
+ Sunglass lenses
The requirements will include some or all of the following attributes
+ surface finish
+ visual quality
+ flame propagation
+ dimensions
+ refractive power of lens
+ prismatic power of lens
+ lens curvature
+ thermal stability
+ optical transmission (UV, visible and near Infra-Red)
+ uniformity of colour
+ colouration limits (red factor, violet factor)
2.8 Cost of frames
-------------------
Varies depending on the type of material, eg plastic frame to titanium metal.
2.9 Cost of lenses
------------------
Like frames varies with the type of the material. Also depends if the
lens requires customised grinding to fit the lens the to the frame.
2.10 Reducing the edge thickness of the lens
--------------------------------------------
Since edge thickness is a function of lens power, centre thickness and
curvature of the front and back curves (all inter-related) the
edge thickness is influenced by the following features:
+ refractive index of the lens material
+ centre thickness (eg safety lenses have a greater central
thickness)
+ size of the spectacle frame, i.e larger eyesize means a
bigger lens.
2.11 Internet Ordering of Prescriptiions and Sunglasses
------------------------------------------------------
The are a wide variety of companies that advertise on the internet for
dispensing of prescriptions and sunglasses. The list is too long
to list here and the reader is advised to use one of the Web search
engines to locate any potential sites.
+============================================================================+
+ Section 3: Contact Lenses +
+============================================================================+
3.0 WWW Sites on Contact Lenses
--------------------------------
URL: http: //www.sna.com/etonline/vision
- currently under construction and has potential for useful information.
3.1 Difference between soft and hard contact lenses
-----------------------------------------------------
Soft lenses are manufactured from a plastic hydrogel polymer,
HydroxyEthylMethacrylate (HEMA) which has a varying water content
(38% - ~70%). Lens size is between 13.00 and 14.50mm. Centre thickness
from ~30um
Hard contact lenses are manufactured from a rigid material,
PolyMethylMethacrylate (PMMA). This material can be combined with other
plastics to increase the oxygen permeability. Lens size is between
8.0mm and 10.00mm. Centre thickness from ~100um.
3.2 How to Read a Contact Lens Description:
-------------------------------------------
A contact lens prescription differs from a spectacle prescription primarily
by the addition of lens parameters. It should be noted that the power of a
contact lens will not necessarily be the same as that of a spectacle
prescription due to the optics of a lens being closer to the eye. The
technical term is "effectivity" of the lens.
The other information that is often given with a contact lens prescription
will be the diameter of the lenses, basecurve, possibly additional curves
lens material, design or manufacturer name.
For example,
OD: 8.6/14.0/-1.00DS
OS: 8.6/14.0/-1.50DS
The data is
BaseCurve: 8.6mm
Diameter: 14.0mm
Power: -1.00D and -1.50
3.2 Types of Soft Contact Lenses
----------------------------------
Soft contact lenses vary depending on either the refractive correction that
they are prescibed for or the design and type of material.
Major types include :
3.2.1 Spherical
-----------------
Single prescription power 360 degrees around the lens.
3.2.2 Toric
-------------
Contains both a spherical and cylinder component to correct prescriptions
which have astigmatism. Lenses may be thicker in one meridian or have
modified thickness profiles to enable the lens to maintain correct orientation
on the eye.
3.2.3 Disposable
------------------
Spherical or toric contact lenses which are designed to be worn for a certain
time period, eg, weekly, two- weekly, monthly. Lenses are generally
sold in a "blister" combination pack, eg 3 months supply with intention
that lenses are "disposed" of at the end of the time period.
3.2.4 Extended Wear
--------------------
Lens manufactured from a high-water content material and/or with a very
thin centre thickness to enable maximum oxygen transmission. Lenses can
be worn overnight, or for a number of days without removal (see
comments on complications for further information).
3.2.5 Bifocal Lenses
---------------------
There are a variety of designs in bifocals, essentially all trying
to provide a transition or reading zone for use at near. The different
designs include;
+ aspheric multifocal -
+ simultaneous vision concentric - has either a central near or
distance zone with surrounding zone of opposite type to centre
(eg, centre near,distance surround)
+ diffraction/holographic - based on diffraction grating principles
3.3 Types of Rigid Contact Lenses
---------------------------------
Rigid contact lenses also vary depending on the material and the design of
the shape of the contact lens. Rigid lenses provide a "new" front surface
to the eye and help in the elimination of astigmatism because the tear film
fills the gap between the lens and the astigmatic cornea.
For information on lens conditions see the URL listed below.
Major Material types include;
3.3.1 PMMA
------------
Original material used in the construction of "hard" contact lenses.
3.3.2 RGP
----------
Combination of PMMA and other polymers to increase the oxygen permeability
and allow longer wearing time. Lenses are also often larger in diameter
than PMMA to increase the comfort of wearing the lens
Major design types include;
3.3.3 Spherical
---------------
same as for soft contact lenses
3.3.4 Toric
-----------
Can be either toric periphery, which is used for fitting reasons, or
bi-toric used to correct residual astigmatism.
3.3.5 Bifocal
-------------
The different types include;
+ multifocal aspheric - light from all distances focus without interruption
by lines or zones of the lens
+ concentric - different zones of the lens for distance and/or near
+ zone based - parallel bands of focal zones for various distances
+ segmented - three basic types
(a) lens divided into two roughly equal parts like an
executive bifocal
(b) lens divided into two unequal parts with near part like
a segment of bofocal spectacles, either flat-top or
crescent shaped
(c) internally fused segment of different refractive index.
Research photos of RGP Lenses on the eye include
URL: http://www.route-one.co.uk/route-one/scotlens/
This site maintains some nice research photos of RGP lenses with conditions
such as
- Drying
- Surface Wetting
- Deposits
- Mucous Deposits
3.4 Costs of Contact Lenses
----------------------------
Varies depending on the type of contact lenses
Lens Type FittingFee MaterialFee Total
In Australia
Spherical Soft ~A$200
Toric Soft ~A$240
Disposable ~A$85 (3months supply)
RGP Spherical ~A$240
In UK:
Spherical Soft ~UK$20
In USA:
Spherical Soft 100 60 160
Toric Soft 155 170 325
Disposable 85 100/6mths 185 (6mths)
RGP Spherical 115 85 200
3.5 Solutions required for CL maintenance
-----------------------------------------
In general all contact lenses, whether soft of rigid type, will
require a cleaning, disinfecting and rinsing solution. Enzymatic agents
(protein remover) may also be required to reduce build up of protein on
the lens surface.
3.6 Common CL Wearer Questions
------------------------------
3.6.1 Why do I have to clean my Contact Lenses ?
--------------------------------------------------
Cleaning removes surface debris and bacteria that may adhere to the contact
lenses.
3.6.2 Why is there a limit to the length of wearing time ?
-----------------------------------------------------------
The cornea the "clear part of the eye", is avascular or without a blood supply.
It is avascular otherwise it wouldn't transmit light without distortion.
As a result of this living tissue being avascular it is necessary to
obtain oxygen from the atmosphere. The wearing of a contact lens interrupts
the flow of oxygen to the cornea and due to changes in the metabolic pump of
the corneal cells the tissue thickens, called oedema.
Contact lenses, as described in Section XXX, are manufactured from material
that allows maximum oxygen transmission. But this is still not exactly the
same as the 20.4% therefore the lens wearing time must be controlled to
reduce oxygen deprivation to the cornea.
3.6.3 Can I lose the lens behind my eye ?
------------------------------------------
Short answer is no. The conjunctiva, the tissue that covers the white part
of the eye (the sclera) forms a cul-de-sac between from the edge of the cornea
to the eyelid margin. Sometimes a contact lens, especially a soft
contact lens may roll up and become difficult to find. An eyecare practitioner
will be able to locate the lens and remove it.
3.6.4 How do I tell if I've lost the lens from my eye?
-------------------------------------------------------
Cover the other eye to determine if vision is still clear from the eye where
you suspect that you've lost the lens. If vision is blurred then more than
likely the lens is either dislodged from the cornea or has fallen from the
eye.
3.6.5 How can I tell if I've put them in the wrong eye ?
---------------------------------------------------------
Check the vision of each eye by covering alternate eyes with your hand.
If the vision is unclear then try swapping the lenses and then recheck the
vision.
3.6.6 How to tell if the lenses are inside out ?
-------------------------------------------------
Soft lenses will appear as a smooth dish shape when placed on the end of a
finger and when the lens is the right way around.
3.6.7 How do I know when to dispose of my disposable lenses ?
-------------------------------------------------------------
The lenses will not automatically self-destruct at the end of the wearing
period :-) The lenses should be discarded when the lens wearing time has
elapsed, as advised by your eyecare professional. This wearing time/period
has been chosen to minimise complications with contact lens wear so it should
be adhered to !
3.6.8 Can I store my lenses in tap water ?
-------------------------------------------
Short answer - No !. There are a number of "nasty" micro- organisms,
especially acanthomeba (sp) which likes feeding on corneal tissue.
Storing lenses in water also defeats the purpose of using a disinfecting
solution as there is no disinfection occurring. If anything you are
exposing your lenses to more potentially dangerous microorganisms.
3.6.9 Can I store my lenses dry ?
----------------------------------
Soft lenses - definitely not !. Soft lenses stored dry will turn into
"corn-flakes" and only go "snap-crackle and pop" when you touch the lens.
Hard lenses should also be stored in a soaking/disinfecting solution to
reduce lens contamination.
3.6.10 How long does it take to adapt to new lenses ?
------------------------------------------------------
Soft lenses are generally worn for 2-4hrs on the first day and the wearing
time is increased by about 2hrs per day, up until 8hrs of daily wear is
achieved. The rate at which the wearing time is increased and the maximum
number of hours that the lenses can be worn will depend on the recommendations
of your eyecare practitioner.
Rigid lenses are generally worn for 2-4hrs on the first day with an increase
of 1-2hrs each day until 8 hours of daily wear is obtained. Again the rate
at which the wearing time is increased and maximum number of hours of
wearing time will depend on the person and the recommendations of your
eyecare practitioner.
3.6.11 Why do I have to use protein cleaner ?
----------------------------------------------
Contact lenses are exposed to a complex environment whilst on the eye.
The tearfilm contains a number of different proteins which potentially
adhere to the surface of the contact lens. The buildup of protein
reduces the wettability of the surface of the contact lens which causes
a "smeary" or "fogginess" to vision. A protein cleaner, often an enzymatic
agent, will help in reducing the potential for protein to adhere to the
contact lens surface. One of the benefits of disposable contact lenses is
that protein cleaners are not necessary because the lenses are disposed of
before the is a large build up of proteinateous material.
3.6.12 Can I use eyedrops with my contact lenses ?
---------------------------------------------------
In general eyedrops shouldn't be used with contact lenses because the lens
can absorb the eye drop and result in a concentrated buildup of the solution.
There are special, "in-eye" lubricants that many manufacturers/pharmaceutical
companies produce for use with contact lenses. Check with your eyecare
professional if any doubts about the solution.
3.6.13 How do I get around dryness with contact lenses ?
---------------------------------------------------------
Try using an "in-eye" lubricant. There can be some dryness if you work in
an air-conditioned environment. If the problem persist consult a professional.
3.7 Risks and Benefits of Contact Lenses
------------------------------------------
Benefits
- no need to wear glasses
- no spectacle scotoma - ie "blind-spot" due to frame edge
- overcome problems of spectacle magnification, especially when large
difference in spectacle prescription between the two eyes.
Risks
- corneal odema
- corneal ulcers
- contact lens induced conjunctivitis
3.8 Mail order contact lenses
-------------------------------
Easiest method to locate the contact lens suppliers on the internet is
to run a Web Search request.
In Aust. this practice is not allowed under current Govt. legislation.
There are a number of sites that offer ordering of contact lenses on the
WWW. Some examples include:
[TBD list of contact lens suppliers on the net ]
3.9 Making your own saline - risks
------------------------------------
Greatest risk in manufacturing your own saline is the acanthamoeba.
Reference:
Phares RE, Microbiology and hygenic care of hydrophilic lenses,
Contacto, 16(3):10-12, 1972
--
Grant Sayer
EMAIL: gra...@research.canon.oz.au PHONE: +61-2-805-2937
SNAIL: Canon Information Systems Research Australia
1 Thomas Holt Drive, North Ryde, Australia 2113